Synthesis of N-[N-(3,3-dimethylbutyl)-L-α-aspartyl]-L-phenylalanine 1-methyl ester using oxazolidinone derivatives

ABSTRACT

Synthesis of N-[N-(3,3-dimethylbutyl)-L-α-aspartyl]-L-phenylalanine 1-methyl ester by treating N-(3,3-dimethylbutyl)-L-aspartic acid with ketones to give oxazolidinone derivatives, which are condensed with L-phenylalanine methyl ester.

This application claims the benefit of U.S. Provisional PatentApplication No. 60/205,694, filed May 19, 2000.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to the synthesis ofN-[N-(3,3-dimethylbutyl)-L-α-aspartyl]-L-phenylalanine 1-methyl ester(neotame) using novel oxazolidinone derivatives. This method ofproducing neotame is an alternative to the conventional synthetic routefor producing neotame which uses aspartame as a starting material.

2. Related Background Art

N-[N-(3,3-dimethylbutyl)-L-α-aspartyl]-L-phenylalanine 1-methyl ester(neotame) is a high potency dipeptide sweetener (about 8000× sweeterthan sucrose) that has the formula

Neotame may be synthesized using a variety of synthetic methods. Thechemical synthesis of neotame is disclosed in U.S. Pat. Nos. 5,480,668,5,510,508, 5,728,862, 6,077,962 and WO 00/15656, the disclosure of eachof which is incorporated by reference herein.

U.S. Pat. Nos. 5,510,508 and 5,728,862 describe the synthesis of neotameby hydrogenation of a mixture of aspartame and 3,3-dimethylbutyraldehydewith a catalyst such as Pd on carbon. This synthesis is represented bythe following equation.

International Patent Publication No. WO 00/15656 describes the formationof neotame by hydrogenation of a mixture of 3,3-dimethylbutyraldehydeand Z-aspartame(N-benzyloxycarbonyl-L-α-aspartyl-L-phenylalanine-1-methyl ester) in amethanolic solvent. U.S. Pat. No. 6,077,962 describes the synthesis ofneotame using a peptide coupling method of an activated derivative ofN-(3,3-dimethylbutyl)-L-aspartic acid and L-phenylalanine orL-phenylalanine methyl ester.

The literature teaches regioselective methods for the formation ofα-aspartyl peptide bonds, as opposed to β-aspartyl peptide bonds, in thesynthesis of aspartame. One of these methods comprises thecyclocondensation of carbonyl compounds with N-protected aspartic acidas described in Chinese Patent CN 11748844 A. The resultingoxazolidinone derivative, having the structure

wherein PG is a protecting group, R¹ is H or R², R² is CX₃ and X is Cl,Br or F, has the α-aspartyl carbonyl as an ester and the β-aspartylcarbonyl as a carboxylic acid. Thus, only the α-aspartyl carbonyl isactivated for peptide bond formation. The nitrogen protecting groupsincluded Z, Boc, formyl and acetyl groups.

Other examples in the literature teach regioselective methods for theformation of α-aspartyl peptide bonds, as opposed to β-aspartyl peptidebonds. Such methods comprise the cyclocondensation of carbonyl compoundswith aspartic acid without nitrogen protection. In these cases, thenitrogen of the oxazolidinone derivative having the structure

is an amine and not part of an amide, as was the case where nitrogen wasprotected.

The treatment of neohexyl-L-aspartic acid with carbonyl compounds toform oxazolidinone derivatives useful in the selective preparation ofneotame without the formation of the beta isomer is not described in theabove-described art.

It would be desirable, however, to develop more efficient andcost-effective methods of preparing high purity neotame from readilyavailable or readily obtainable materials.

SUMMARY OF THE INVENTION

The present invention relates to the synthesis ofN-[N-(3,3-dimethylbutyl)-L-α-aspartyl]-L-phenylalanine 1-methyl estervia novel oxazolidinone derivatives.

According to the present inventive method, neotame is synthesized byreacting N-(3,3-dimethylbutyl)-L-aspartic acid and a ketone in a solventfor a time and at a temperature sufficient to produce an oxazolidinonederivative and by reacting the oxazolidinone derivative andphenylalanine or phenylalanine methyl ester in the solvent for a timeand at a temperature sufficient to produceN-[N-(3,3-dimethylbutyl)-L-α-aspartyl]-L-phenylalanine 1-methyl ester.

DETAILED DESCRIPTION

The present invention relates to the regioselective formation ofN-alkylated α-aspartyl amides via the use of ketones, and particularlyto the use of such regioselective processing to obtain oxazolidinonederivatives which can react with L-phenylalanine methyl ester in asolvent with or without acid and/or a catalyst to yieldN-[N-(3,3-dimethylbutyl)-L-α-aspartyl]-L-phenylalanine 1-methyl ester(neotame) with the usual work-up. The present synthetic method isrepresented by the following reaction scheme:

wherein R¹ is R², R² is Ph or CX₃, X is H, Cl, Br or F, R³ and R⁴ takentogether is ═O, or R³ and R⁴ are the same and are OCH₃ or OC₂H₅

According to the present invention, neotame is synthesized by reactingN-(3,3-dimethylbutyl)-L-aspartic acid and a ketone in a first solventfor a time and at a temperature sufficient to produce an oxazolidinonederivative and by reacting the oxazolidinone derivative andphenylalanine or phenylalanine methyl ester in a second solvent for atime and at a temperature sufficient to produceN-[N-(3,3-dimethylbutyl)-L-α-aspartyl]-L-phenylalanine 1-methyl ester.

According to the first step of the present inventive method, anadmixture of N-(3,3-dimethylbutyl)-L-aspartic acid and a ketone arereacted in a first solvent for a time and at a temperature sufficient toproduce an oxazolidinone derivative.

Ketones of the formula R¹R²C═O or acetals of ketones of the formulaCR¹R²R³R⁴, wherein R¹ is R², R² is Ph or CX₃, X is H, Cl, Br or F, R³and R⁴ taken together is ═O, or R³ and R⁴ are the same and are OCH₃ orOC₂H₅ are suitable for use in the present invention. Ph is phenyl orsubstituted phenyl. Suitable ketones include, without limitation,hexafluoroacetone, 1,1,1-trifluoroacetone, hexachloroacetone, andcombinations thereof.

N-(3,3-dimethylbutyl)-L-aspartic acid is prepared as described in U.S.Pat. No. 6,077,962, the disclosure of which is incorporated by referenceherein. The ketones are readily available starting materials. TheN-(3,3-dimethylbutyl)-L-aspartic acid and the ketone are typicallycombined in a molar ratio ranging from about 1:1 to about 1:4.

The solvents suitable for use as the first solvent in the presentinvention are limited only by reactivity considerations; in other words,the solvent must not react with the oxazolidinone derivative, thephenylalanine nucleophile or the resulting product, thereby impeding orprohibiting the desired reaction. Suitable solvents include, withoutlimitation, tetrahydrofuran, diethyl ether, t-butyl methyl ether, ethylacetate, dioxane, toluene, butyl acetate, methyl acetate,dichloromethane, dimethylformamide, dimethylsulfoxide and combinationsthereof.

Generally, the time sufficient to produce an oxazolidinone derivativeranges from about 1 to about 48 hours, preferably from about 2 to about24 hours. Generally, the temperature sufficient to produce neotameaccording to the present invention ranges from about 20° C. to about150° C., preferably from about 22° C. to about 70° C.

In certain embodiments of the present invention, a catalyst may bepresent during the reaction of N-(3,3-dimethylbutyl)-L-aspartic acid andthe ketone. Suitable catalysts include, without limitation,p-toluenesulfonic acid. In certain embodiments of the present invention,an acid may be present during the reaction ofN-(3,3-dimethylbutyl)-L-aspartic acid and the ketone. Suitable acidsinclude, without limitation, formic acid, acetic acid, p-toluenesulfonicacid, methane sulfonic acid, 10-camphorsulfonic acid and combinationsthereof.

According to the second step of the present inventive method, anadmixture of the oxazolidinone derivative and phenylalanine orL-phenylalanine methyl ester are reacted in a second solvent for a timeand at a temperature sufficient to produceN-[N-(3,3-dimethylbutyl)-L-α-aspartyl]-L-phenylalanine 1-methyl ester.

L-phenylalanine methyl ester is a readily available starting material.Typically, the L-phenylalanine methyl ester is used in a molar ratiowith the oxazolidinone derivative produced in the first step of thepresent invention ranging from about 1:1 to about 1:2.

Generally, the time sufficient to produceN-[N-(3,3-dimethylbutyl)-L-α-aspartyl]-L-phenylalanine 1-methyl esterranges from about 1 to about 48 hours, preferably from about 12 to about24 hours. Generally, the temperature sufficient to produce neotameaccording to the present invention ranges from about 0° C. to about 50°C., preferably from about 22° C. to about 40° C.

The solvents suitable for use as the second solvent in the presentinvention are limited only by reactivity considerations; in other words,the solvent must not react with the oxazolidinone derivative, thephenylalanine nucleophile or the resulting product, thereby impeding orprohibiting the desired reaction. Suitable solvents include, withoutlimitation, tetrahydrofuran, diethyl ether, t-butyl methyl ether, ethylacetate, dioxane, toluene, butyl acetate, methyl acetate,dichloromethane, dimethylformamide, dimethylsulfoxide and combinationsthereof. In certain embodiments of the present invention, the firstsolvent and the second solvent used in the first and second steps,respectively, are the same solvent.

The present invention may also include additional steps. Such additionalsteps include, without limitation, solvent concentration adjustment,seeding, cooling (crystallization), and neotame isolation.

Typically crystallization of neotame is accomplished by cooling themixture to about 0-25° C., preferably to about 5-10° C., over the courseof about 0.5-2 hours, preferably about 1-2 hours.

Seeding prior to or during crystallization can initiate a controlledcrystal growth rate according to the present invention. Hence, thereaction mixture may optionally be seeded in an amount from 0.0001%-10%,by weight of the N-[N-(3,3-dimethylbutyl)-L-α-aspartyl]-L-phenylalanine1-methyl ester in the solution, preferably from 0.1% to 1% and mostpreferably from 0.1% to 0.5%. Seeding is typically performed at 25-35°C. and preferably at 28-30° C.

The reaction mixture may be unstirred or stirred while neotamecrystallizes according to the present invention.

Crystallized neotame may be separated from the solvent solution by avariety of solid-liquid separation techniques that utilize centrifugalforce, that include, without limitation, vertical and horizontalperforated basket centrifuge, solid bowl centrifuge, decantercentrifuge, peeler type centrifuge, pusher type centrifuge, Heinkel typecentrifuge, disc stack centrifuge and cyclone separation. Additionally,separation may be enhanced by any of pressure, vacuum, and gravityfiltration methods, that include, without limitation, the use of belt,drum, nutsche type, leaf, plate, Rosenmund type, sparkler type, and bagfilters and filter press. Operation of the neotame solid-liquidseparation device may be continuous, semi-continuous or in batch mode.The neotame solid may also be washed on the separation device usingvarious liquid solvents, including, without limitation, water, methanoland mixtures thereof. The neotame solid can also be partially andtotally dried on the separation device using any number of gases,including, without limitation, nitrogen and air, to evaporate residualliquid solvent. The neotame solid may be automatically or manuallyremoved from the separation device using liquids, gases or mechanicalmeans by either dissolving the solid or maintaining the solid form.

The neotame synthesized according to the present invention may bepurified by any known method including, but not limited to, thefollowing methods. U.S. Pat. No. 5,728,862 outlines a purificationmethod by which neotame is precipitated out of an aqueous/organicsolvent solution, wherein the aqueous/organic solvent solution has anamount of organic solvent of about 17% to about 30% by weight. CopendingU.S. patent application Ser. No. 09/448,671, filed on Nov. 24, 1999,relates to methods of purifying neotame by crystallization in a varietyof organic solvent/aqueous organic solvent mixtures; each of thesemethods involves the use of an organic solvent and water mixture andsolvent distillation. Copending U.S. patent application Ser. No.09/449,314, filed on Nov. 24, 1999, relates to methods of purifyingneotame using chromatography.

The neotame synthesized according to the present invention is themonohydrate, which may be dried to produce an anhydrous form.

The crystallized and isolated neotame solid may be further purified by avariety of drying methods. Such methods are known to those skilled inthe art and include, but are not limited to, the use of a rotary vacuumdryer, fluid bed dryer, rotary tunnel dryer, plate dryer, tray dryer,Nauta type dryer, spray dryer, flash dryer, micron dryer, pan dryer,high and low speed paddle dryer and microwave dryer.

The Examples which follow are intended as an illustration of certainpreferred embodiments of the invention, and no limitation of theinvention is implied.

EXAMPLE 1

4-Carbomethoxy-3-N-(3,3-dimethylbutyl)-2,2-bis(trifluoro-methyl)oxazolidin-5-one

A gas flow of hexafluoroacetone is blown at a moderate rate at roomtemperature onto an intensely stirred suspension of 100 mmol ofN-(3,3-dimethylbutyl)-L-aspartic acid in 40 ml anhydrousdimethylsulfoxide. The absorption of the gas starts after a shortinduction period. The gas flow is adjusted in a way that an excess ofhexafluoroacetone is always present but that any condensation ofhexafluoroacetone at a CO₂-radiator is avoided. The end of the reactionis recognized by the beginning backflow of hexafluoroacetone. A clearsuspension is formed. After further stirring for 2-3 hours, the reactionsolution will be decanted onto 200 ml of ice water and extracted threetimes with 100 ml ethyl acetate each time. In order to remove anyremaining dimethylsulfoxide and hexafluoroacetone hydrate, the combinedsolutions of ethyl acetate are washed three times with 50 ml ice waterand dried over anhydrous sodium sulfate. After removal of the solventunder reduced pressure, the compounds crystallize out.

EXAMPLE 2

N-[N-(3,3-dimethylbutyl)-L-α-aspartyl]-L-phenylalanine 1-methyl ester

To a solution of 20 mmol of4-carbomethoxy-3-N-(3,3-dimethylbutyl)-2,2-bis(trifluoromethyl)oxazolidin-5-onein 50 ml anhydrous diethyl ether, a solution of 4.25 g (24 mmol)L-phenylalanine methyl ester in 5 ml anhydrous diethyl ether is addeddropwise with stirring at room temperature. A crystalline solidsubstance begins to precipitate within a few minutes. The reactionfinishes within 24 hours.

EXAMPLE 3

2-[(4S)-3-(3,3-dimethylbutyl)-5-oxo-2,2-bis(trifluoromethyl)-1,3-oxazolan-4-yl]aceticacid

A gas flow of hexafluoroacetone is blown at a moderate rate at roomtemperature onto an intensely stirred suspension of 10 mmol ofN-(3,3-dimethylbutyl)-L-aspartic acid in 20 ml 1,4-dioxane. A clearsolution is formed overnight. The solvent was removed in vacuo, and theoily residue was confirmed to be an almost quantitative amount of2-[(4S)-3-(3,3-dimethylbutyl)-5-oxo-2,2-bis(trifluoromethyl)-1,3-oxazolan-4-yl]aceticacid by NMR.

EXAMPLE 4

N-[N-(3,3-dimethylbutyl)-L-α-aspartyl]-L-phenylalanine 1-methyl ester

2-[(4S)-3-(3,3-dimethylbutyl)-5-oxo-2,2-bis(trifluoromethyl)-1,3-oxazolan-4-yl]aceticacid (2 mmol) and L-phenylalanine 1-methyl ester (2 mmol) were dissolvedin tetrahydrofuran (15 ml). The mixture was stirred at room temperaturefor 24 hours. The solvent was removed in vacuo to yield an oil. A whitesolid, confirmed to be neotame by NMR, was obtained after stirring theoil in water overnight. Neotame was obtained in 90% yield.

EXAMPLE 5

N-[N-(3,3-dimethylbutyl)-L-α-aspartyl]-L-phenylalanine 1-methyl ester

L-phenylalanine 1-methyl ester hydrochloride (10 mmol), tetrahydrofuran(15 ml) and sodium acetate (NaOAc, 10 mmol) were loaded into a 50 mlflask. The mixture was stirred at room temperature for 15 minutes. Asolution of2-[(4S)-3-(3,3-dimethylbutyl)-5-oxo-2,2-bis(trifluoromethyl)-1,3-oxazolan-4-yl]aceticacid (10 mmol) in tetrahydrofuran (10 ml) was added to the mixture. Themixture was then stirred at room temperature for 24 hours. The solventwas removed in vacuo to yield a residue. The residue was stirred inwater overnight at room temperature. The precipitated solid wasfiltered, washed with water and dried to yield neotame in 90% yield.

EXAMPLE 6

2-[(4S)-3-(3,3-dimethylbutyl)-2,2dimethyl-5-oxo-1,3-oxazolan-4-yl]aceticacid.

Other variations and modifications of this invention will be obvious tothose skilled in this art. This invention is not to be limited except asset forth in the following claims.

1. A process of synthesizingN-[N-(3,3-dimethylbutyl)-L-α-aspartyl]-L-phenylalanine 1-methyl estercomprising the steps of: (a) reacting an admixture of (i)N-(3,3-dimethylbutyl)-L-aspartic acid and (ii) a ketone or an acetal ofa ketone in a solvent for a time and at a temperature sufficient toproduce an oxazolidinone derivative; and (b) reacting an admixture ofthe oxazolidinone derivative and L-phenylalanine or L-phenylalaninemethyl ester in the solvent for a time and at a temperature sufficientto produce N-[N-(3,3-dimethylbutyl)-L-α-aspartyl]-L-phenylalanine1-methyl ester.
 2. The process according to claim 1, wherein the ketoneis selected from the group consisting of hexafluoroacetone,hexachloroacetone, and combinations thereof.
 3. The process according toclaim 1, wherein the acetal of a ketone is selected from the groupconsisting of dimethyl acetal of hexafluoroacetone, diethyl acetal ofhexafluoroacetone, dimethyl acetal of hexachloroacetone, diethyl acetalof hexachloroacetone, and combinations thereof.
 4. The process accordingto claim 1, wherein the solvent is selected from the group consisting oftetrahydrofuran, diethyl ether, t-butyl methyl ether, ethyl acetate,dioxane, toluene, butyl acetate, methyl acetate, dichloromethane,dimethylformamide, dimethylsulfoxide and combinations thereof.
 5. Theprocess according to claim 1, wherein the ratio ofN-(3,3-dimethylbutyl)-L-aspartic acid to the ketone or the acetal of aketone is from about 1:1 to about 1:4.
 6. The process according to claim1, wherein the temperature sufficient to produce the oxazolidinonederivative is from about 20° C. to about 150° C.
 7. The processaccording to claim 6, wherein the temperature sufficient to produce theoxazolidinone derivative is from about 22° C. to about 70° C.
 8. Theprocess according to claim 1, wherein the time sufficient to produce theoxazolidinone derivative is from about 1 hour to about 48 hours.
 9. Theprocess according to claim 8, wherein the time sufficient to produce theoxazolidinone derivative is from about 12 hours to about 24 hours. 10.The process according to claim 1, wherein the admixture ofN-(3,3-dimethylbutyl)-L-aspartic acid and a ketone or an acetal of aketone further comprises a catalyst.
 11. The process according to claim10, wherein the catalyst is p-toluenesulfonic acid.
 12. The processaccording to claim 1, wherein the admixture ofN-(3,3-dimethylbutyl)-L-aspartic acid and a ketone or an acetal of aketone further comprises an acid.
 13. The process according to claim 12,wherein the acid is selected from the group consisting of formic acid,acetic acid, p-toluenesulfonic acid, methane sulfonic acid andcombinations thereof.
 14. The process according to claim 1, wherein theratio of L-phenylalanine or L-phenylalanine methyl ester to theoxazolidinone derivative is from about 1:1 to about 1:2.
 15. The processaccording to claim 1, wherein the temperature sufficient to produceN-[N-(3,3-dimethylbutyl)-L-α-aspartyl]-L-phenylalanine 1-methyl ester isfrom about 0° C. to about 50° C.
 16. The process according to claim 15,wherein the temperature sufficient to produceN-[N-(3,3-dimethylbutyl)-L-α-aspartyl]-L-phenylalanine 1-methyl ester isfrom about 22° C. to about 40° C.
 17. The process according to claim 1,wherein the time sufficient to produceN-[N-(3,3-dimethylbutyl)-L-α-aspartyl]-L-phenylalanine 1-methyl ester isfrom about 1 hour to about 48 hours.
 18. The process according to claim17, wherein the time sufficient to produceN-[N-(3,3-dimethylbutyl)-L-α-aspartyl]-L-phenylalanine 1-methyl ester isfrom about 12 hours to about 24 hours.